1. Field of the Invention
The present invention relates to an optical information recording medium, and particularly to an optical information recording medium in which information is recorded, reproduced and rewritten by changing an optical property of a light absorbing layer thereof by irradiating the light absorbing layer with a laser light beam. More particularly, the present invention relates to an optical recording medium which can be used as a CD-ROM or a DVD-ROM such as CD-R media, CD-RW media, DVD-R media, DVD-RW media, DVD+R media and DVD+RW. In addition, the present invention also relates to an information recording method using the optical information recording medium.
2. Discussion of the Related Art
Various optical information recording media capable of recording, reproducing and erasing information upon irradiation of laser light thereto have been developed and used. Specific examples of such optical information recording media include magneto-optical media, CD-R media, CD-RW media, DVD-R media, DVD-RW media, DVD+R media, DVD+RW media and DVD-ROM media. Now, there is a need to record a larger amount of information at a higher speed. Therefore, a need exists for optical information recording media which can perform high density recording at high linear recording speed.
In attempting to satisfy the need, light reflection layers which include a material having both high reflectance and high heat conductivity, such as Au, Ag, Cu and Al, have been investigated. In particular, there are great expectations for a light reflection layer including Ag which has the highest reflectance and heat conductivity among metals.
When a light reflection layer including Ag is used for optical information recording media, the recording media have the following advantages:                (A) the recording medium increases has high light reflectance over a broad wavelength range;        (B) signal amplitude of the resultant medium can be increased because Ag has good optical characteristics;        (C) when the light reflection layer is used for a phase-change optical recording medium, the number of overwriting can be increased if the light reflection layer is made by a rapid cooling method;        (D) when the light reflection layer is used for a phase-change optical recording medium, recording speed (linear speed) can be increased if the light reflection layer is made by a rapid cooling method;        (E) productivity can be increased because Ag has high sputtering efficiency; and        (F) since the sputtering time taken for forming the light reflection can be decreased, the quantity of heat stress applied to the recording medium can be decreased, resulting in improvement of the mechanical properties of the resultant recording medium.        
However, recording media having a light reflection layer including Ag have the following drawbacks:                (G) the light reflection layer is easily corroded under high temperature and high humidity conditions;        (H) the light reflection layer is easily corroded by sulfur and chlorine;        (I) adhesion of the light reflection layer with a layer on which the reflection layer is formed is relatively small;        (J) since Ag is a noble metal, the light reflection layer is more expensive than a layer made of Al, etc.; and        (K) when a substrate having a thickness of 0.6 mm is used, the resultant recording medium has abnormal mechanical properties due to membrane stress of the Ag film.        
In order to prevent corrosion of Ag reflection layers, the following methods have been disclosed.    (1) a silver alloy, AgCu, is used for the light reflection layer (Japanese Laid-Open Patent Publication No. 57-186244);    (2) a silver alloy, AgMg, is used for the light reflection layer (Japanese Laid-Open Patent Publication No. 7-3363);    (3) a silver alloy, AgOM (M represents Sb, Pd or Pt), is used for the light reflection layer (Japanese Laid-Open Patent Publication No. 9-156224); and    (4) a silver alloy, AgPdCu, is used for the light reflection layer (Japanese Laid-Open Patent Publication No. 2000-285517).
In addition, Japanese Patent No. 2,749,080 discloses a light reflection layer which includes Ag and another element such as Ti, V, Fe, Co, Ni, Zn, Zr, Nb, Mo, Rh, Pd, Sn, Sb, Te, Ta, W, Ir, Pt, Pb, Bi and C, to control the heat conductivity of the light reflection layer.
However, when CD-R media, CD-RW media, DVD-RW media, DVD+R media and DVD+RW media are prepared while these materials are used as their reflection layer and information is recorded therein, the recorded signals have unsatisfactory reflectance and signal amplitude.
This is because when such a metal is added to Ag in an amount not less than 1% by weight, Ag loses its advantages, i.e., high reflectance and high heat conductivity.
In addition, when the archival high temperature preservation reliability of the above-mentioned recording media is evaluated, it is found that reproduction errors increase if the media are preserved for 300 hours under an environmental condition of 80° C. and 85% RH. Namely, the recording media have unsatisfactory preservation reliability.
Further, when such light reflection layers are used for laminated recording media such as DVD+RW media and the substrates have a thickness as thin as 0.6 mm, the substrates tend to deform due to heat stress and internal stress of the light reflection layers, and thereby an information recording/reproducing problem occurs.